Various types of hearing prostheses provide people having different types of hearing loss with the ability to perceive sound. Hearing loss may be conductive, sensorineural, or some combination of both conductive and sensorineural. Conductive hearing loss typically results from a dysfunction in any of the mechanisms that ordinarily conduct sound waves through the outer ear, the eardrum, or the bones of the middle ear. Sensorineural hearing loss typically results from a dysfunction in the inner ear, including the cochlea, where sound vibrations are converted into neural signals, or any other part of the ear, auditory nerve, or brain that processes the neural signals.
People with some forms of hearing loss may benefit from hearing prostheses, such as acoustic hearing aids or vibration-based hearing aids. An acoustic hearing aid typically includes a small microphone to detect sound, an amplifier to amplify certain portions of the detected sound, and a small speaker to transmit the amplified sound into the person's ear. Vibration-based hearing aids typically include a small microphone to detect sound, and a vibration mechanism to apply vibrations corresponding to the detected sound to a person's bone, thereby causing vibrations in the person's inner ear, thus bypassing the person's auditory canal and middle ear. Vibration-based hearing aids include bone anchored hearing aids, direct acoustic cochlear devices, or other vibration-based devices (e.g. bone-conduction hearing glasses and vibration-based behind-the-ear prostheses), and may be partially or totally implanted or simply in external contact with a suitable body part of the person.
One type of bone conduction device utilizes a surgically-implanted mechanism to transmit sound via direct vibrations of an implant recipient's skull. A component of the bone conduction device detects sound waves, which are converted into a series of stimulation signals delivered to the implant recipient's skull bones via an electromechanical stimulator (e.g., a mechanical actuator).
By providing stimulation to the recipient's skull, the bone conduction device effectively bypasses the recipient's middle ear and auditory canal, which is advantageous for recipients having medical conditions that affect the middle or outer ear. The vibrations of the recipient's skull bones cause fluid motion within the recipient's cochlea, thereby enabling the recipient to perceive sound based on the vibrations. Similarly, a direct acoustic cochlear device typically utilizes a surgically-implanted mechanism to transmit sound by directly moving the ossicular chain of the recipient, which causes fluid motion within the recipient's cochlea. Other non-surgical vibration-based hearing aids use similar vibration mechanisms to transmit sound via direct vibration of a recipient's teeth or other cranial or facial bones.
Each type of hearing prosthesis has an associated sound processor. In some types of hearing prostheses, the sound processor amplifies sounds received by the prosthesis. However, other types of hearing prosthesis include a more advanced processor. For example, some processors are programmable and include advanced signal processing functions (e.g., noise reduction functions) and speech algorithms.
In some hearing prosthesis systems, prostheses are present on both the left and right sides of the recipient. In such a bilateral system, the left prosthesis provides audio corresponding to the left ear and the right prosthesis provides audio corresponding to the right ear. The two prostheses may operate independently of each other. However, in some systems, the two prostheses can communicate with one another and transfer the captured audio or data from the left ear prosthesis to the right ear prosthesis and vice versa. Yet other systems may include more than two prostheses in communication with one another.
Some example bilateral hearing prosthesis systems include a vibration mechanism or stimulator in each prosthesis that outputs an amplified captured sound as mechanical vibrations. In these systems, a first vibration-based hearing prosthesis is coupled to the left side of a recipient's head and a second vibration-based hearing prosthesis is coupled to the right side of a recipient's head. Feedback occurs when a portion of the sound captured by the microphone associated with one of the vibration-based hearing prostheses includes either (i) the mechanical vibrations produced by the vibration stimulator of the respective vibration-based hearing prosthesis or (ii) the mechanical vibrations produced by the vibration stimulator of the other vibration-based hearing prosthesis. When the microphone of one of the prostheses captures the mechanical vibrations from either of the two prostheses and then the respective prosthesis produces an output based on those vibrations, an undesirable acoustic feedback results.
For example, the left vibration-based hearing prosthesis receives a sound and responsively provides a stimulus to the recipient. The right vibration-based hearing prosthesis may receive both (i) a second sound and (ii) a portion of the stimulus provided by the left vibration-based hearing prosthesis. The right vibration-based hearing prosthesis then responsively creates a second stimulus based on the combination of both (i) the second sound and (ii) the portion of the stimulus provided to the recipient by the left vibration-based hearing prosthesis and captured by the microphone of the right vibration-based hearing prosthesis. The feedback loop may continue if the left vibration-based hearing prosthesis then receives a portion of the second stimulus (created by the right vibration-based hearing prosthesis). When fitting a bilateral system, the conventional practice is for the audiologist to reduce the prescribed gain for each unit by around 3 dB, to prevent the recipient from hearing excessive loudness.